The conventional practice of plastic working a steel is broadly classified into cold working and hot working. Cold working is mainly applied to the manufacture of a small diameter or thin-walled product, while hot working is mainly applied to the manufacture of a large diameter or thick-walled product. As the strength of a steel with a large diameter or thick wall becomes greater, its working becomes more difficult.
Thus, for example, a conventional coil spring of more than 10 mm in diameter is manufactured by hot working a steel into the form of a coil spring and then quenching and tempering it to impart high strength to the spring.
Hot working is a method of easily forming a coil spring, but it has various drawbacks. The spring surface is liable to be broken because of hot working under lowered strength of the material at high temperature, and decarburization is liable to occur with heating to the austenitizing temperature. Furthermore, strength variance is liable to occur because of the heat treatment applied to the spring in the form of a coil, and heat treatment also causes a coarse surface or deformation, yielding a finished product which is more liable to be defective than a cold worked product.
Cold working of a coil spring from a relatively small diameter, for example, oil-tempered wire with a required strength, in which the wire is not heated for working, retains the strength of the wire and causes no coarse surface. In this respect, cold working may be said to be superior to hot working, but the trouble is that, when the wire has a high strength, its working becomes harder as its diameter increases.